assertTrue(headMap instanceof SynchronizedSortedMap);
    assertThat(((SynchronizedSortedMap<String, Integer>) headMap).mutex).isSameInstanceAs(mutex);
  }

  public void testHeadMap_k_b() {
    NavigableMap<String, Integer> map = create();
    NavigableMap<String, Integer> headMap = map.headMap("a", true);
    assertTrue(headMap instanceof SynchronizedNavigableMap);